[Federal Register Volume 78, Number 244 (Thursday, December 19, 2013)]
[Proposed Rules]
[Pages 76775-76779]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2013-30236]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 25

[Docket No. FAA-2013-1002; Notice No. 25-13-36-SC]


Special Conditions: Airbus, Model A350-900 Series Airplane; 
Lightning Protection of Fuel Tank Structure To Prevent Fuel Tank Vapor 
Ignition

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Notice of proposed special conditions.

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SUMMARY: This action proposes special conditions for the Airbus Model 
A350-900 series airplanes. These airplanes will have a novel or unusual 
design feature(s) that will incorporate a nitrogen generation system 
(NGS) for all fuel tanks that actively reduce flammability exposure 
within the fuel tanks significantly below that required by the fuel 
tank flammability regulations. Among other benefits, the NGS 
significantly reduces the potential for fuel vapor ignition caused by 
lightning strikes. The applicable airworthiness regulations do not 
contain adequate or appropriate safety standards for this design 
feature. These proposed special conditions contain the additional 
safety standards that the Administrator considers necessary to 
establish a level of safety equivalent to that established by the 
existing airworthiness standards.

DATES: Send your comments on or before February 3, 2014.

ADDRESSES: Send comments identified by docket number FAA-2013-1002 
using any of the following methods:
     Federal eRegulations Portal: Go to http://www.regulations.gov/and follow the online instructions for sending your 
comments electronically.
     Mail: Send comments to Docket Operations, M-30, U.S. 
Department of Transportation (DOT), 1200 New Jersey Avenue SE., Room 
W12-140, West Building Ground Floor, Washington, DC, 20590-0001.
     Hand Delivery or Courier: Take comments to Docket 
Operations in Room W12-140 of the West Building Ground Floor at 1200 
New Jersey Avenue SE., Washington, DC, between 8 a.m. and 5 p.m., 
Monday through Friday, except federal holidays.
     Fax: Fax comments to Docket Operations at 202-493-2251.
    Privacy: The FAA will post all comments it receives, without 
change, to http://www.regulations.gov/, including any personal 
information the commenter provides. Using the search function of the 
docket Web site, anyone can find and read the electronic form of all 
comments received into any FAA docket, including the name of the 
individual sending the comment (or signing the comment for an 
association, business, labor union, etc.). DOT's complete Privacy Act 
Statement can be found in the Federal Register published on April 11, 
2000 (65 FR 19477-19478), as well as at http://DocketsInfo.dot.gov/ gov/.
    Docket: Background documents or comments received may be read at 
http://www.regulations.gov/at any time. Follow the online instructions 
for accessing the docket or go to the Docket Operations in Room W12-140 
of the West Building Ground Floor at 1200 New Jersey Avenue SE., 
Washington, DC, between 9 a.m. and 5 p.m., Monday through Friday, 
except federal holidays.

FOR FURTHER INFORMATION CONTACT: Doug Bryant, Propulsion/Mechanical 
Systems, ANM-112, Transport Airplane Directorate, Aircraft 
Certification Service, 1601 Lind Avenue SW., Renton, Washington, 98057-
3356; telephone (425) 227-2384; facsimile (425) 227-1320.

SUPPLEMENTARY INFORMATION: 

Comments Invited

    We invite interested people to take part in this rulemaking by 
sending written comments, data, or views. The most helpful comments 
reference a specific portion of the special conditions, explain the 
reason for any recommended change, and include supporting data.
    We will consider all comments we receive on or before the closing 
date for comments. We may change these special conditions based on the 
comments we receive.

Background

    On August 25, 2008, Airbus applied for a type certificate for their 
new Airbus Model A350-900 series airplane. Later, Airbus requested and 
the FAA approved an extension to the application for FAA type 
certification to June 28, 2009. The Model A350-900 series has a 
conventional layout with twin wing-mounted Rolls-Royce Trent XWB 
engines. It features a twin aisle 9-abreast economy class layout, and 
accommodates side-by-side placement of LD-3 containers in the cargo 
compartment. The basic Model A350-900 series configuration will 
accommodate 315 passengers in a standard two-class arrangement. The 
design cruise speed is Mach 0.85 with a Maximum Take-Off Weight of 
602,000 lbs. Airbus proposes the Model A350-900 series to be certified 
for extended operations (ETOPS) beyond 180 minutes at entry into 
service for up to a 420-minute maximum diversion time.

Type Certification Basis

    Under Title 14, Code of Federal Regulations (14 CFR) 21.17, Airbus 
must show that the Model A350-900 series meets the applicable 
provisions of 14 CFR part 25, as amended by Amendments 25-1 through 25-
129.
    If the Administrator finds that the applicable airworthiness 
regulations (i.e., 14 CFR part 25) do not contain adequate or 
appropriate safety standards for the Model A350-900 series because of a 
novel or unusual design feature, special conditions are prescribed 
under Sec.  21.16.

[[Page 76776]]

    In addition to the applicable airworthiness regulations and special 
conditions, the Model A350-900 series must comply with the fuel vent 
and exhaust emission requirements of 14 CFR part 34 and the noise 
certification requirements of 14 CFR part 36 and the FAA must issue a 
finding of regulatory adequacy under Sec.  611 of Public Law 92-574, 
the ``Noise Control Act of 1972.''
    The FAA issues special conditions, as defined in 14 CFR 11.19, 
under Sec.  11.38, and they become part of the type-certification basis 
under Sec.  21.17(a)(2).
    Special conditions are initially applicable to the model for which 
they are issued. Should the type certificate for that model be amended 
later to include any other model that incorporates the same novel or 
unusual design feature, the special conditions would also apply to the 
other model under Sec.  21.101.

Novel or Unusual Design Features

    The Airbus Model A350-900 series will incorporate the following 
novel or unusual design features: fuel tank NGS that is intended to 
control fuel tank flammability for all fuel tanks. This NGS is designed 
to provide a level of performance that applies the more stringent 
standard for warm day flammability performance applicable to normally 
emptied tanks within the fuselage contour from Sec.  25.981(b) and 14 
CFR part 25 appendix M, to all fuel tanks of the Model A350-900 series. 
This high level of NGS performance for all fuel tanks is an unusual 
design feature not envisioned at the time the regulations in the Model 
A350-900 series certification basis were promulgated.

Discussion

    The certification basis of the Airbus Model A350-900 series 
includes Sec.  25.981, as amended by Amendment 25-125, as required by 
14 CFR 26.37. This amendment includes the ignition prevention 
requirements in Sec.  25.981(a), as amended by Amendment 25-102, and it 
includes revised flammability limits for all fuel tanks and new 
specific limitations on flammability for all fuel tanks as defined in 
Sec.  25.981(b), as amended by Amendment 25-125.

Ignition Source Prevention

    Section 25.981(a)(3) requires applicants to show that an ignition 
source in the fuel tank system could not result from any single 
failure, from any single failure in combination with any latent failure 
condition not shown to be extremely remote, or from any combination of 
failures not shown to be extremely improbable. This requirement was 
originally adopted in Amendment 25-102 and it requires the assumption 
that the fuel tanks are always flammable when showing that the 
probability of an ignition source being present is extremely remote. 
(Amendment 25-102 included Sec.  25.981(c) that required minimizing 
fuel tank flammability and this was defined in the preamble as being 
equivalent to unheated aluminum fuel tanks located in the wing.) This 
requirement defines three types of scenarios that must be addressed in 
order to show compliance with Sec.  25.981(a)(3). The first scenario is 
that any single failure, regardless of the probability of occurrence of 
the failure, must not cause an ignition source. The second scenario is 
that any single failure, regardless of the probability of occurrence, 
in combination with any latent failure condition not shown to be at 
least extremely remote, must not cause an ignition source. The third 
scenario is that any combination of failures not shown to be extremely 
improbable must not cause an ignition source. Demonstration of 
compliance with this requirement would typically require a structured, 
quantitative safety analysis. Design areas that have latent failure 
conditions typically would be driven by these requirements to have 
multiple fault tolerance, or ``triple redundancy.'' This means that 
ignition sources are still prevented even after two independent 
failures.

Flammability Limits

    Section 25.981(b) states that no fuel tank fleet average 
flammability exposure may exceed 3 percent of the flammability exposure 
evaluation time calculated using the method in part 25, Appendix N, or 
the fleet average flammability of a fuel tank within the wing of the 
airplane being evaluated, whichever is greater. If the wing is not a 
conventional unheated aluminum wing, the analysis must be based on an 
assumed equivalent conventional construction unheated aluminum wing. In 
addition, for fuel tanks that are normally emptied during operation and 
that have any part of the tank located within the fuselage contour, the 
fleet average flammability for warm days (above 80 [deg]F) must be 
limited to 3 percent as calculated using the method in part 25, 
Appendix M.

Application of Existing Regulations Inappropriate Due to Impracticality

    Since the issuance of Sec.  25.981(a)(3), as amended by Amendment 
25-102, the FAA has conducted certification projects in which 
applicants found it impractical to meet the requirements of that 
regulation for some areas of lightning protection for fuel tank 
structure. Partial exemptions were issued for these projects. These 
same difficulties exist for the Airbus Model A350-900 series project.
    The difficulty of designing multiple-fault-tolerant structure, and 
the difficulty of detecting failures of hidden structural design 
features in general, makes compliance with Sec.  25.981(a)(3) uniquely 
challenging and impractical for certain aspects of the electrical 
bonding of structural elements. Such bonding is needed to prevent 
occurrence of fuel tank ignition sources from lightning strikes. The 
effectiveness and fault tolerance of electrical bonding features for 
structural joints and fasteners is partially dependent on design 
features that cannot be effectively inspected or tested after assembly 
without damaging the structure, joint, or fastener. Examples of such 
features include a required interference fit between the shank of a 
fastener and the hole in which the fastener is installed, metal foil or 
mesh imbedded in composite material, a required clamping force provided 
by a fastener to pull two structural parts together, and a required 
faying surface bond between the flush surfaces of adjacent pieces of 
structural material such as in a wing skin joint or a mounting bracket 
installation. In addition, other features that can be physically 
inspected or tested may be located within the fuel tanks; therefore, it 
is not practical to inspect for failures of those features at short 
intervals. Examples of such failures include separation or loosening of 
cap seals over fastener ends and actual structural failures of internal 
fasteners. This inability to practically detect manufacturing errors 
and failures of structural design features critical to lightning 
protection results in degraded conditions that occur and remain in 
place for a very long time, possibly for the remaining life of the 
airplane.
    Accounting for such long failure latency periods in the system 
safety analysis required by Sec.  25.981(a)(3) would require multiple 
fault tolerance in the structural lightning protection design. As part 
of the design development activity for the Model A350-900 series, 
Airbus has examined possible design provisions to provide multiple 
fault tolerance in the structural design to prevent ignition sources 
from occurring in the event of lightning attachment to the airplane in 
critical locations. Airbus has concluded from this examination that 
providing multiple fault tolerance for some structural elements is not 
practical. Airbus has also identified some areas of

[[Page 76777]]

the Model A350-900 series design where it is impractical to provide 
even single fault tolerance in the structural design to prevent 
ignition sources from occurring in the event of lightning attachment 
after a single failure. The FAA has reviewed this examination with 
Airbus in detail and has agreed that providing fault tolerance beyond 
that in the proposed Model A350-900 series design for these areas would 
be impractical.
    As a result of the Airbus Model A350-900 series and other 
certifications projects, the FAA has now determined that compliance 
with Sec.  25.981(a)(3) is impractical for some areas of lightning 
protection for fuel tank structure, and that application of Sec.  
25.981(a)(3) to those design areas is therefore inappropriate. The FAA 
plans further rulemaking to revise Sec.  25.981(a)(3). As appropriate, 
the FAA plans to issue special conditions or exemptions, for 
certification projects progressing before the revision is complete. 
This is discussed in FAA Memorandum ANM-112-08-002, Policy on Issuance 
of Special Conditions and Exemptions Related to Lightning Protection of 
Fuel Tank Structure, dated May 26, 2009.\1\
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    \1\ The memorandum may be viewed at: http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgPolicy.nsf/0/12350AE62D393B7A862575C300709CA3?OpenDocument&Highlight=anm-112-08-002.
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Application of Existing Regulations Inappropriate Due to Compensating 
Feature That Provides Equivalent Level of Safety

    Section 25.981(b) sets specific standards for fuel tank 
flammability as discussed above under ``Flammability Limits.'' Under 
that regulation, the fleet average flammability exposure of all fuel 
tanks on the Model A350-900 series may not exceed 3 percent of the 
flammability exposure evaluation time calculated using the method in 
part 25, Appendix N, or the fleet average flammability of a wing main 
tank within an equivalent construction conventional unheated aluminum 
wing fuel tank, whichever is greater. The typical fleet average fuel 
tank flammability of fuel tanks located in the wing ranges between 1 
and 5 percent. If it is assumed that an Model A350-900 series 
equivalent conventional unheated aluminum wing fuel tank would not 
exceed a fleet average flammability time of 3 percent, the actual 
composite airplane wing fuel tank design would be required to comply 
with the 3 percent fleet average flammability standard and therefore a 
means to reduce the flammability to 3 percent would be required. 
However, the proposed Model A350-900 series design includes NGS for all 
fuel tanks that will also be shown to meet the additional, more 
stringent warm day average flammability standard in part 25, Appendix 
M, which is only required for normally emptied fuel tanks with some 
part of the tank within the fuselage contour. Fuel tanks that meet this 
requirement typically have average fuel tank flammability levels well 
below the required 3 percent.
    Since the proposed NGS for all fuel tanks on the Model A350-900 
series provides performance that meets part 25, Appendix M, the FAA has 
determined that the risk reduction provided by this additional 
performance will provide compensation for some relief from the ignition 
prevention requirements of Sec.  25.981(a)(3) while still establishing 
a level of safety equivalent to that established in the regulations.
    In determining the appropriate amount of relief from the ignition 
prevention requirements of Sec.  25.981(a), the FAA considered the 
original overall intent of Amendment 25-102, which was to ensure the 
prevention of catastrophic events due to fuel tank vapor explosion. 
These proposed special conditions are intended to achieve that 
objective through a prescriptive requirement that fault tolerance (with 
respect to the creation of an ignition source) be provided for all 
structural lightning protection design features where providing such 
fault tolerance is practical, and through a performance-based standard 
for the risk due to any single failure vulnerability that exists in the 
design. In addition, for any structural lightning protection design 
features for which Airbus shows that providing fault tolerance is 
impractical, these proposed special conditions would require Airbus to 
show that a fuel tank vapor ignition event due to the summed risk of 
all non-fault-tolerant design features is extremely improbable. Airbus 
would be required to show that this safety objective is met by the 
proposed design using a structured system safety assessment similar to 
that currently used for demonstrating compliance with Sec. Sec.  25.901 
and 25.1309.
    Given these novel design features, and the compliance challenges 
noted earlier in this document, the FAA has determined that application 
of Sec.  25.981(a)(3) is inappropriate in that it is neither practical 
nor necessary to apply the ignition source prevention provisions of 
Sec.  25.981(a)(3) to the specific fuel tank structural lightning 
protection features of the Airbus Model A350-900 series airplanes. 
However, without the Sec.  25.981(a)(3) provisions, the remaining 
applicable regulations in the Model A350-900 series certification basis 
would be inadequate to set an appropriate standard for fuel tank 
ignition prevention. Therefore, in accordance with provisions of Sec.  
21.16, the FAA has determined that, instead of Sec.  25.981(a)(3), 
alternative fuel tank structural lighting protection requirements be 
applied to fuel tank lightning protection features that are integral to 
the airframe structure of the Model A350-900 series. These proposed 
alternative requirements are intended to provide the level of safety 
intended by Sec.  25.981(a)(3), based on our recognition, as discussed 
above, that a highly effective NGS for the fuel tanks makes it 
unnecessary to assume that the fuel tank is always flammable. As 
discussed previously, the assumption that the fuel tanks are always 
flammable was required when demonstrating compliance to the ignition 
prevention requirements of Sec.  25.981(a)(3).
    One resulting difference between these proposed special conditions 
and the Sec.  25.981(a)(3) provisions they are meant to replace is the 
outcome being prevented--fuel vapor ignition versus an ignition source. 
These proposed special conditions acknowledge that the application of 
fuel tank flammability performance standards will reduce fuel tank 
flammability to an extent that it is appropriate to consider the 
beneficial effects of flammability reduction when considering design 
areas where it is impractical to comply with Sec.  25.981(a)(3).
    One of the core requirements of these proposed special conditions 
is a prescriptive requirement that structural lightning protection 
design features must be fault tolerant. (An exception wherein Airbus 
can show that providing fault tolerance is impractical, and associated 
requirements, is discussed below.) The other core requirement is that 
Airbus must show that the design, manufacturing processes, and 
airworthiness limitations section of the instructions for continued 
airworthiness include all practical measures to prevent, and detect and 
correct, failures of structural lightning protection features due to 
manufacturing variability, aging, wear, corrosion, and likely damage. 
The FAA has determined that, if these core requirements are met, a fuel 
tank vapor ignition event due to lightning is not anticipated to occur 
in the life of the airplane fleet. This conclusion is based on the fact 
that a critical lightning strike to any given airplane is itself a 
remote event, and on the fact that fuel tanks must be shown to be 
flammable for only a relatively

[[Page 76778]]

small portion of the fleet operational life.
    For any non-fault-tolerant features proposed in the design, Airbus 
must show that eliminating these features or making them fault tolerant 
is impractical. The requirements and considerations for showing it is 
impractical to provide fault tolerance are described in FAA Memorandum 
ANM-112-08-002. This requirement is intended to minimize the number of 
non-fault tolerant features in the design.
    For areas of the design where Airbus shows that providing fault 
tolerant structural lighting protection features is impractical, non-
fault-tolerant features will be allowed provided Airbus can show that a 
fuel tank vapor ignition event due to the non-fault-tolerant features 
is extremely improbable when the sum of probabilities of those events 
due to all non-fault-tolerant features is considered. Airbus will be 
required to submit a structured, quantitative assessment of fleet 
average risk for a fuel tank vapor ignition event due to all non-fault-
tolerant design features included in the design. This will require 
determination of the number of non-fault tolerant design features, 
estimates of the probability of the failure of each non-fault-tolerant 
design feature, and estimates of the exposure time for those failures. 
This analysis must include failures due to manufacturing variability, 
aging, wear, corrosion, and likely damage.
    It is acceptable to consider the probability of fuel tank 
flammability, the probability of a lightning strike to the airplane, 
the probability of a lightning strike to specific zones of the airplane 
(for example, Zone 2 behind the nacelle, but not a specific location or 
feature), and a distribution of lightning strike amplitude in 
performing the assessment provided the associated assumptions are 
acceptable to the FAA. The analysis must account for any dependencies 
among these factors, if they are used. The assessment must also account 
for operation with inoperative features and systems, including any 
proposed or anticipated dispatch relief. This risk assessment 
requirement is intended to ensure that an acceptable level of safety is 
provided given the non-fault-tolerant features in the proposed design.
    Part 25, Appendix N, as adopted in Amendment 25-125, in conjunction 
with these proposed special conditions, constitutes the standard for 
how to determine flammability probability. In performing the safety 
analysis required by these special conditions, relevant Sec.  
25.981(a)(3) compliance guidance is still applicable. Appropriate 
credit for the conditional probability of environmental or operational 
conditions occurring is normally limited to those provisions involving 
multiple failures, and this type of credit is not normally allowed in 
evaluation of single failures. However, these special conditions would 
allow consideration of the probability of occurrence of lightning 
attachment and flammable conditions when assessing the probability of 
structural failures resulting in a fuel tank vapor ignition event.
    The FAA understands that lightning protection safety for airplane 
structure is inherently different from lightning protection for 
systems. We intend to apply these proposed special conditions only to 
structural lightning protection features of fuel systems. We do not 
intend to apply the alternative standards used under these proposed 
special conditions to other areas of the airplane design evaluation.

Requirements Provide Equivalent Level of Safety

    In recognition of the unusual design feature discussed above, and 
the impracticality of requiring multiple fault tolerance for lightning 
protection of certain aspects of fuel tank structure, the FAA has 
determined that an equivalent level of safety to direct compliance with 
Sec.  25.981(a)(3) will be achieved for the Model A350-900 series by 
applying these proposed requirements. The FAA considers that, instead 
of only concentrating on fault tolerance for ignition source 
prevention, significantly reducing fuel tank flammability exposure in 
addition to preventing ignition sources is a better approach to 
lightning protection for the fuel tanks. In addition, the level of 
average fuel tank flammability achieved by compliance with these 
proposed special conditions is low enough that it is not appropriate or 
accurate to assume in a safety analysis that the fuel tanks may always 
be flammable.
    Section 25.981(b), as amended by Amendment 25-125, sets limits on 
the allowable fuel tank flammability for the Model A350-900 series. 
Paragraph 2(a) of these proposed special conditions applies the more 
stringent standard for warm day flammability performance applicable to 
normally emptied tanks within the fuselage contour from Sec.  25.981(b) 
and part 25, Appendix M, to all of the fuel tanks of the Model A350-900 
series.
    Because of the more stringent fuel tank flammability requirements 
in these proposed special conditions, and because the flammability 
state of a fuel tank is independent of the various failures of 
structural elements that could lead to an ignition source in the event 
of lightning attachment, the FAA has agreed that it is appropriate in 
this case to allow treatment of flammability as an independent factor 
in the safety analysis. The positive control of flammability and the 
lower flammability that is required by these proposed special 
conditions exceeds the minimum requirements of Sec.  25.981(b). This 
offsets a reduction of the stringent standard for ignition source 
prevention in Sec.  25.981(a)(3), which assumes that the fuel tank is 
flammable at all times.
    Given the stringent requirements for fuel tank flammability, the 
fuel vapor ignition prevention and the ignition source prevention 
requirements in these proposed special conditions will prevent `` . . . 
catastrophic failure . . . due to ignition of fuel or vapors'' as 
stated in Sec.  25.981(a). Thus, the overall level of safety achieved 
by these proposed special conditions is considered equivalent to that 
which would be required by compliance with Sec.  25.981(a)(3) and (b).

Applicability

    As discussed above, these proposed special conditions apply to 
Airbus Model A350-900 series airplanes. Should Airbus apply later for a 
change to the type certificate to include another model incorporating 
the same novel or unusual design feature, the special conditions would 
apply to that model as well.

Conclusion

    This action affects only certain novel or unusual design features 
on the Airbus Model A350-900 series airplanes. It is not a rule of 
general applicability.

List of Subjects in 14 CFR Part 25

    Aircraft, Aviation safety, Reporting and recordkeeping 
requirements.

    The authority citation for these special conditions is as follows:

    Authority: 49 U.S.C. 106(g), 40113, 44701, 44702, 44704.

The Proposed Special Conditions

    Accordingly, the Federal Aviation Administration (FAA) proposes the 
following special conditions as part of the type certification basis 
for Airbus Model A350-900 series airplanes.

1. Definitions

    Most of the terms used in these proposed special conditions, 
Alternative Fuel Tank Structural Lightning Protection Requirements, 
either have the common dictionary meaning or are defined in Advisory 
Circular 25.1309-1A, System Design and Analysis, dated June 21, 1988. 
The following definitions

[[Page 76779]]

are the only terms intended to have a specialized meaning when used in 
these proposed special conditions:
    (a) Basic Airframe Structure. Includes design elements such as 
structural members, structural joint features, and fastener systems 
including airplane skins, ribs, spars, stringers, etc., and associated 
fasteners, joints, coatings, and sealant. Basic airframe structure may 
also include those structural elements that are expected to be removed 
for maintenance, such as exterior fuel tank access panels and fairing 
attachment features, provided maintenance errors that could compromise 
associated lightning protection features would be evident upon an 
exterior preflight inspection of the airplane and would be corrected 
prior to flight.
    (b) Permanent Systems Supporting Structure. Includes static, 
permanently attached structural parts (such as brackets) that are used 
to support system elements. It does not include any part intended to be 
removed, or any joint intended to be separated, to maintain or replace 
system elements or other parts, unless that part removal or joint 
separation is accepted by the FAA as being extremely remote.
    (c) Manufacturing Variability. Includes tolerances and variability 
allowed by the design and production specifications as well as 
anticipated errors or escapes from the manufacturing and inspection 
processes.
    (d) Extremely Remote. Conditions that are not anticipated to occur 
to each airplane during its total life, but which may occur a few times 
when considering the total operational life of all airplanes of one 
type. Extremely remote conditions are those having an average 
probability per flight hour on the order of 1 x 10-7 or 
less, but greater than on the order of 1 x 10-9.
    (e) Extremely Improbable. Conditions that are so unlikely that they 
are not anticipated to occur during the entire operational life of all 
airplanes of one type. Extremely improbable conditions are those having 
an average probability per flight hour of the order of 1 x 
10-9 or less.

2. Alternative Fuel Tank Structural Lightning Protection Requirements

    For lightning protection features that are integral to fuel tank 
basic airframe structure or permanent systems supporting structure, as 
defined in this these proposed special conditions, Definitions, for 
which Airbus shows and the FAA finds compliance with Sec.  25.981(a)(3) 
to be impractical, the following requirements may be applied in lieu of 
the requirements of Sec.  25.981(a)(3):
    (a) Airbus must show that the airplane design meets the 
requirements of part 25, Appendix M, as amended by Amendment 25-125, 
for all fuel tanks installed on the airplane.
    (b) Airbus must show that the design includes at least two 
independent, effective, and reliable lightning protection features (or 
sets of features) such that fault tolerance to prevent lightning-
related ignition sources is provided for each area of the structural 
design proposed to be shown compliant with these proposed special 
conditions in lieu of compliance with the requirements of Sec.  
25.981(a)(3). Fault tolerance is not required for any specific design 
feature if:
    (1) For that feature, providing fault tolerance is shown to be 
impractical, and
    (2) Fuel tank vapor ignition due to that feature and all other non-
fault-tolerant features, when their fuel tank vapor ignition event 
probabilities are summed, is shown to be extremely improbable.
    (c) Airbus must perform an analysis to show that the design, 
manufacturing processes, and airworthiness limitations section of the 
instructions for continued airworthiness include all practical measures 
to prevent, and detect and correct, failures of structural lightning 
protection features due to manufacturing variability, aging, wear, 
corrosion, and likely damage.

    Issued in Renton, Washington, on November 15, 2013.
John P. Piccola, Jr.,
Manager, Transport Airplane Directorate, Aircraft Certification 
Service.
[FR Doc. 2013-30236 Filed 12-18-13; 8:45 am]
BILLING CODE 4910-13-P